1. Field of the Invention
The present invention relates to a process for producing droplets with a narrow size distribution from liquids. The term liquid used in connection with the invention covers both clear liquids, such as solutions, and for instance metal melts and flowable dispersions, like for instance suspensions.
2. Description of the Prior Art
Producing droplets from liquids is often termed atomization. Common atomizing processes used commercially at a large scale are spraying under pressure in single-fluid pressure nozzles, for instance hollow cone nozzles, spraying with a gas in two-fluid-nozzles or pneumatic atomization, and the atomization with rotary atomizers. The invention also relates to processes according to the last-mentioned principle.
In many technical processes a narrow droplet size distribution is desired. This is, inter alia, because spray drying plants must be dimensioned according to the biggest droplets of the spray, as these droplets require the longest residence time in the drier. Thus a broad droplet spectre means in spite of a lower average droplet size large and consequently unfavourable dimensions. The smallest droplets in the spray necessitate high costs for cleaning the discharge air in form of filters and cyclones or the like devices. A broad droplet size spectrum moreover leads to a broad particle size distribution of the produced spray-dried powder and consequently in some cases to undesirable technical properties.
Up until now all known atomization processes which are used technically at a large scale, i.e. a production capacity of more than 100 kg/h, produce droplets with comparatively broad size spectres. See for instance Chem.--Ing. Techn. 62 (1990) 12, pp. 983-994.
Admittedly, droplets with a fairly narrow size distribution may be obtained with rotary atomizers of commonly used design. Thereby, the effect of the laminar jet disintegration is utilized. If the liquid is delivered to the centre of a plane round rotating disc, it flows, when a certain limited liquid production is kept, as a laminar film radially outwards and forms at the trailing edges of the disc threads of liquid. The liquid threads are created at the periphery of the trailing edge in a natural way with regular spacing. The subsequent disintegration of the liquid threads results in droplets with a very narrow size spectre. If the size distribution of the droplets talus attained is described, for instance with the RRSB function according to DIN 66 141, then an evenness parameter of 6&lt;m&lt;8 is attained. As average droplet size d.sub.v.50 is in the present text the droplet diameter defined, at which the 50%-value of the volume distribution is obtained; i.e. that 50% of the sprayed liquid volume get smaller droplet diameters--and 50% of the sprayed liquid get a bigger droplet diameter than d.sub.v.50.
A considerable drawback of the atomizing method with plane rotating discs is that the amount of liquid passing this flow area is very small. As an estimate, the passing amount V of low-viscous liquids will lie in the range of 0.21&lt;V(.rho..sup.3 n.sup.2 /D.sup.3 .sigma..sup.3).sup.0.25 &lt;0.32. D is the disc diameter, .rho. the liquid density, .sigma. the surface tension of the liquid, and n the speed of rotation. Both the narrow limits of the throughput range and the low value of the throughput of liquid hinder widespread use of this process.
In order to obtain a higher throughput it has been suggested to arrange several discs over each other, Chem.--Ing.--Techn. 36 (1964) 1, pages. 52-59. A uniform distribution of the liquid on the discs is however difficult to obtain with a device that does not easily clog. The narrow throughput range is also in this connection a drawback.
Recently, discs or cups have been used, which at their periphery are provided with evenly spaced notches or grooves, for spraying of lacquers. In this way the throughput range can be broadenend while maintaining in this connection a drawback.
Recently, discs or cups have been used, which at their periphery are provided with evenly spaced notches or grooves, for spraying of lacquers. In this way the throughput range can be broadenend while maintaining laminar jet formation. However, also with this embodiment the throughput range is insufficient for many technical purposes.
FR A-2 662 374 discloses an atomizer rotor which is capable of working with varying volume, a homogenous atomization being obtained even of high-viscous liquids. This atomizer rotor is on the outside provided with grooves, to which the liquid to be atomized is supplied through perforations in the cylindrical wall of the rotor. It is stated that the length of these perforations must never exceed their double diameter. The liquid to be atomized is distributed on the inner side of the rotor by a stationary tube. It seems in particular to be grooves arranged on the exterior of the rotor that are to ensure an even droplet size at the atomization, and the advantage obtained is limited.
The atomizer normally used for spray drying consists of a, low, cylindrical body, most frequently called atomizer wheel, said body having bores or ducts. The diameter of the bores are normally in the range of 5-30 mm. The liquid is often supplied centrally and flows radially outwards and leaves the atomizer through the bores. The design has admittedly the advantage that the comparatively big bores normally do not clog, but the throughput for large scale technical uses is chosen so high that the liquid leaves the bores in thick turbulent jets. Due to the high relative speed between the liquid and the surrounding gas the liquid jets which already leave the openings turbulently are dispersed. Thereby droplets are created at high rotational speed which is necessary in respect of small droplet dimensions, said droplets having a very broad size spectre. At the same time a considerable wear of the walls of the bores occurs when suspensions are atomized on account of the high flow speed.